Cobb, an outfielder for Class A Stockton, tested positive for amphetamines in violation of the Minor League drug program, according to The Associated Press.
The 23-year-old was batting .221 for the Ports with three homers and five RBIs in 68 at-bats. He also had 25 at-bats with Triple-A Sacramento, hitting .240 with two RBIs in 10 games.
Cobb was originally drafted by the A's in the 27th round of the 2006 First-Year Player Draft.
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.